初學Android，遇到這個問題還不知道怎麼辦，Android沒有提供函式來計算，而且自己資料庫用的是MySQL，也是自己不熟悉的資料庫，就沒有過多的想用SQL來寫。

在網上找了幾個，感覺有些複雜，想了想有一點思路，寫出來可以實現

思路：

1. 首先判斷是否日期一前一後
2. 邊計數邊增加比較早的日期
3. 等到兩個日期一樣時，停止操作，返回計數

/**
 * Created by Harry on 2015/4/27.
 * This class is used for compute days between two different dates
 */
public class DateCompute {

    private static String TAG = DateCompute.class.getName();

    private static Calendar startDate;
    private static Calendar endDate;
    private static int dayCount;

    public DateCompute(Calendar p_startDate, Calendar p_endDate) {
        startDate = p_startDate;
        endDate = p_endDate;
        dayCount = 0;
    }

    /**
     * Since both are Calendar, there is no need to add one more month each.
     * @return
     */
    public int computeCalendar(){
        /*if startDate is before endDate, loop continues*/
        while(startDate.before(endDate)){
            startDate.add(Calendar.DAY_OF_MONTH, 1);
            dayCount++;
        }
        Log.v(TAG, "$$$$$$ days between two dates are : " + dayCount);
        return dayCount;
    }


}
 

注意：

1. Calendar類中，MONTH是比真實日期小1的，所以傳遞引數的時候注意變換
2. 如果遇到String型別的，要先轉換成Calendar，下面的程式碼可以實現

方法也就是取substring，然後分別賦值給Calendar物件（資料全是資料庫裡提取的資料，資料庫函式就沒有貼上了）

    private static void calendarMinus(){
        Calendar calendar1 = Calendar.getInstance();
        Calendar calendar2 = Calendar.getInstance();

        String sql = "select startDate, endDate from habit where userID = 2 and habitID = 2;";
        Cursor cursor = Database.getDatabase().getWritableDatabase().rawQuery(sql, null);
        String start, end;
        cursor.moveToFirst();

        start = cursor.getString(cursor.getColumnIndex("startDate"));
        end = cursor.getString(cursor.getColumnIndex("endDate"));
        /*yyyy-MM-dd*/
        calendar1.set(Calendar.YEAR, Integer.parseInt(start.substring(0,4)));
        calendar1.set(Calendar.MONTH, Integer.parseInt(start.substring(5,7)));
        calendar1.set(Calendar.DAY_OF_MONTH, Integer.parseInt(start.substring(8,10)));

        calendar2.set(Calendar.YEAR, Integer.parseInt(end.substring(0,4)));
        calendar2.set(Calendar.MONTH, Integer.parseInt(end.substring(5,7)));
        calendar2.set(Calendar.DAY_OF_MONTH, Integer.parseInt(end.substring(8,10)));

        DateCompute compute = new DateCompute(calendar1, calendar2);
        compute.computeCalendar();
    }